tests: Added a couple tests to make sure that normal operators can work with dtype=object.
Comment.
tests: testOpPrecomputedInput: Fixed to accomodate a change in OpPixelOperator.
operators: generic: OpMultiArrayStacker: AxisIndex is optional if the axis flag already exists in the input.
roi.py: getIntersection: Allow roiA and roiB to be non-intersecting, in which case None is returned.
Comment formatting.